1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Art
Access points are used to provide wireless connectivity to one or more mobile units in a wireless communication system. Exemplary access points may include base stations, base station routers, Access Serving Networks (ASNs), WiMAX routers, and the like. Mobile units may include cellular telephones, personal data assistants, smart phones, text messaging devices, laptop computers, desktop computers, and the like. The access point also provides connectivity to one or more outside networks. For example, in a wireless network that operates according to an IEEE 802.16 protocol, a mobile unit may establish a wireless connection with a WiMAX router, which may include one or more Access Serving Network (ASN) entities and one or more base stations. The WiMAX router may be connected to one or more Connectivity Serving Networks (CSN) that provides connectivity to an outside network.
Security associations may be established and maintained to allow secure communications between mobile units and the serving network. For example, systems that operate according to the IEEE 802.16e and/or WiMAX standards may use the Privacy and Key Management, version 2, (PKMv2) protocol with Extensible Authentication Protocol (EAP) for user authentication and device authorization. The PKMv2 protocol supports device authorization and user authentication between a mobile unit and a home Network Service Provider (NSP) using a three-party scheme.
The three parties in the PKMv2 protocol are the supplicant, the authenticator, and the authentication server. A supplicant is an entity at one end of a point-to-point link that is being authenticated by an authenticator attached to the other end of that link. An authenticator is an entity at one end of a point-to-point link that facilitates authentication of supplicants that may be attached to the other end of the point-to-point link. The authenticator enforces authentication before allowing the supplicant access to services in the network. An authentication server is an entity that provides an authentication service to an authenticator and a supplicant. This authentication server uses the credentials provided by the supplicant to determine whether the supplicant is authorized to access the services provided via the authenticator. For example, in a WiMAX system, the supplicant is the mobile unit, the authenticator resides in the Access Serving Network (ASN), and the authentication server is implemented in an authentication, authorization, and accounting (AAA) server in the Connectivity Serving Network (CSN).
The Extensible Authentication Protocol (EAP) is an encapsulation protocol used to transport packet data units (PDUs) that may be used to negotiate an authentication method between the supplicant and the authentication server. The Extensible Authentication Protocol may be encapsulated within other protocols such as the PKMv2 protocol, the 802.16 protocol, a RADIUS or DIAMETER protocol, a Universal Datagram Protocol (UDP), a Transmission Control Protocol (TCP), an Internet Protocol (IP), and the like. The RADIUS protocol and possibly the DIAMETER protocol are the de facto transport protocols for EAP over IP networks between the authenticator and authentication server. The Extensible Authentication Protocol (EAP) supports cryptographically strong key-deriving methods such as EAP-TLS, EAP-AKA and EAP-MSCHAPv2, as well as reuse of user credential types across WiMAX networks.
Secure connections are typically established according to a security model that specifies an operational relationship between the supplicant, the authenticator, and the authentication server. For example, a four phase security model may be used. In the first phase, a supplicant (e.g., a mobile unit) discovers one or more available base stations that can provide wireless connectivity in a coverage area and selects a particular base station as a preferred (or serving) base station. The mobile unit then discovers configuration data, and the discovery may occur statically and/or dynamically. In the second phase, the supplicant presents its credentials to the authenticator, which forwards the supplicant's credentials to the authentication server. Depending on the authentication method being negotiated, multiple roundtrip communications between the various entities may be used. If the authentication procedure succeeds, the authentication server forwards a session-related key to the authenticator in the third phase. The authentication server also forwards information that may be used to generate the session-related key to the supplicant. The session-related keys held by the authenticator and the supplicant are used to establish a security association manifested by a pair of secret symmetric keys, which may be used to generate keys to protect data transmitted in the fourth phase.
In systems that operate according to the IEEE 802.16 and WiMAX standards, a symmetric key called the Master Key (MK) is pre-provisioned into the supplicant and the authentication server upon initialization of the supplicant's subscription. The Master Key represents the current subscription-based security association and only the supplicant and the authentication server can possess Master Key, which demonstrates authorization to make a decision on behalf of supplicant. An example of a Master Key is the root key used in authentication and key agreement (AKA) protocols. The supplicant and/or the authentication server can generate a Master Session Key (MSK) and/or an Extended Master Session Key (EMSK) from the Master Key. The Master Session Key is typically used for fixed subscribers and the Extended Master Session Key is typically used for mobile subscribers. These keys may be derived as recommended in section 7.10 of the IETF RFC-3748 “Extensible Authentication Protocol.”
The supplicant and the authentication server may derive an AAA-key based on the Master Session Key (or the Extensible Master Session Key). The authentication server populates the AAA-Key into the corresponding authenticator using, for example, the RADIUS and/or DIAMETER protocols to establish a security association between the supplicant, the authenticator, and the authentication server. The supplicant and the authenticator each generate one of a pair of secret symmetric keys, which may be referred to as Pairwise Master Keys (PMKs), using the AAA-key. The IEEE 802.16 and WiMAX standards state that the supplicant and the authenticator derive the Pairwise Master Keys by truncating the AAA-key. Generation of the Pairwise Master Keys marks the successful completion of the Credential Verification and User Authentication phase, i.e. the second phase described above.
The supplicant and the authenticator may each generate a copy of an Authorization Key (AK) using the Pairwise Master Key. For example, the Authorization Key may be computed from the Pairwise Master Key using a pseudo-random function (prf) transform of Base Station and Mobile Station identifiers (BS_ID and MS_ID, respectively):AKi=prf(PMK, BS—ID, MS—ID, . . . )Accordingly, the Authorization Key used by the supplicant and the authenticator remains the same as long as the supplicant remains in contact with the same base station and uses the same Pairwise Master Key. However, the IEEE 802.16e and WiMAX draft standards state that the Pairwise Master Key should not be shared with a (less trusted) target access serving network when a supplicant (e.g., a mobile unit) hands-off from a base station in a source access serving network to a base station in the target access serving network. Instead, the target access serving network should generate a new Pairwise Master Key by executing an Initial Entry EAP authentication process, as described above, which requires accessing the authentication server, e.g., the Home AAA Server.
The supplicant does not know that the access serving network has changed when the supplicant hands off from a base station in the source access serving network to a base station in the target access serving network. Consequently, the supplicant does not know that the current value of the Pairwise Master Key will not be shared with the target access serving network. Since the supplicant cannot distinguish between contiguous entry and initial entry into the target access serving network, the supplicant does not know that the authenticator associated with the new base station in the target access network is different from the initial authenticator, and that the new authenticator does not possess the current Pairwise Master Key. Therefore, following the inter-system handoff, the supplicant will continue to compute the Authorization Key based on the Pairwise Master Key from the source access serving network, the supplicant's MS_ID, and the BS_ID of the new target base station.
To preserve the communication link between the supplicant and the target base station during a handoff, the source access serving network may provide an Authorization Key to the target base station, but this key will also be computed based on the Pairwise Master Key from the source access serving network, the supplicant's MS_ID, and the BS_ID of the new target base station. Providing security key material to the less trusted target base station and/or target access serving network may increase security risks associated with the supplicant, the target base station, and/or the target access serving network. For example, an adversary in the less trusted access serving network may have an opportunity to determine the value of the Pairwise Master Key from the Authorization Key, particularly if the Pairwise Master Key and, therefore, the Authorization Key are used for a relatively long period of time. Once the value of the old Pairwise Master Key has been determined, the adversary may be able to decrypt communications associated with the current session and any older sessions that utilized the same key material.
To prevent this potential security risk, the source access serving network may not provide a copy of the Pairwise Master Key. Instead, the supplicant and the target access serving network may be required to negotiate a new security association after the handoff into an untrusted serving network. For example, the supplicant and the target access serving network may invoke the complete EAP authentication procedure with the Home AAA Server (HAAA), as described above. However, as discussed above, the supplicant typically does not know that it has entered a new (less trusted) target access serving network. Thus, the supplicant may not know that it is necessary to negotiate the new security association. Moreover, the complete EAP procedure typically requires a lengthy exchange that may include multiple transactions, and so the complete EAP procedure may be unreliable in a hand-off region due to the poor link conditions during the hand off. Accordingly, renegotiating a security association during the handoff may increase the likelihood that the communication link is dropped during the handoff.